The present invention relates to an air intake silencer for attenuating the noise of an air flow into the intake of an aspirated machine. Particularly, but not exclusively, the invention relates to a turbocharger compressor inlet silencer.
A conventional exhaust gas turbocharger comprises a compressor driven by a turbine which is itself driven by exhaust gas flow from a reciprocating engine, normally an internal combustion engine. The compressor wheel rotates in a housing to draw in air through an inlet passage and deliver compressed air through an outlet passage to the air intake of the engine. For instance, in an axial compressor the compressor inlet passage is a tubular passage extending from the compressor wheel housing and the outlet passage is an annular volute surrounding the compressor wheel.
It is conventional to fit an inlet silencer to the compressor wheel inlet to attenuate the sound waves produced as air accelerates into the compressor. A typical compressor inlet silencer comprises a cylindrical or conical housing which has a generally annular inlet flow through the walls of the housing and a generally axial outlet flow to the compressor inlet. Noise reduction is achieved by the provision of sound deadening baffles within the silencer housing. Conventional compressor inlet silencer baffles are annular members i.e. disc like with a axial central opening, arranged axially along the axis of the silencer housing (and thus the axis of the compressor wheel) so that the air flowing through the silencer initially flows in a generally radial direction along flow paths defined between adjacent baffles and then turns axially towards the silencer outlet and compressor inlet. The baffles provide a sound attenuating surface which significantly reduces the noise of the air flow into the compressor. The baffles may, for instance, have a composite construction comprising an annular steel plate with cork or felt glued to each side.
Generally silencers are constructed as a combined air silencer/filter module. For instance, a filter membrane may be supported around the silencer housing which defines the annular inlet into the silencer so that the air flow is filtered as it passes radially into the silencer housing.
With any compressor it is important that the intake air flow through the compressor inlet is smooth to enhance the efficiency of the compressor. A problem with conventional compressor inlet silencers as described above is that the merging of the generally radial silencer inlet flow with the generally axial silencer outlet flow produces turbulence in the downstream air supplied to the compressor wheel.
An example of such a combined compressor inlet silencer/filter is disclosed in U.S. Pat. No. 4,204,586. This shows two alternative arrangements of annular baffles of the general type described above. In a first arrangement the baffles each lie in a plane perpendicular to the axis of the silencer whereas in the second embodiment the baffles have a frusto-conical configuration so that their noise attenuating surfaces lie at an angle to the axis of the silencer. In each case the inner circumference of the baffles curve inwardly towards the axial silencer outlet in an attempt to deflect the generally radial inlet flows into the axial outlet flow. In addition, the separation of the baffles, which are arranged equidistantly along the axis of the silencer, is such that the sum of the cross-sections of the partial flow passage as defined between the baffles is approximately equal to the average flow cross-section of the silencer manifold delivering air to the compressor inlet. Both of these features are provided to promote smooth merging of the radial air flow with the axial silencer outlet flow to reduce turbulence both within the silencer housing and in the air stream fed to the compressor.
It is an object of the present invention to provide a silencer design which further reduces turbulence in the air flow from the silencer to the intake of the downstream machine (such as a compressor or the like) and which has improved sound attenuating characteristics.
According to a first aspect of the present invention there is provided a silencer for connection to the air intake of a machine, the silencer comprising:
a housing having an axis, an outlet aperture being defined at one axial end of the housing;
a plurality of axially spaced annular noise attenuating baffles, each baffle having an outer circumference and an inner circumference defining a central aperture, the central apertures of each baffle collectively defining an axial outlet flow passage to said outlet aperture;
the baffles defining a series of axially spaced generally annular partial flow passages such that air flowing through said silencer is initially split between said flow passages and then merges into the axial outlet passage;
wherein each of the annular flow passages curves radially inwards from its outer to its inner circumference in a direction towards the axial outlet.
According to a second aspect of the present invention there is provided a silencer for connection to the air intake of a machine, the silencer comprising:
a housing having an axis, an outlet aperture being defined at one axial end of the housing;
a plurality of axially spaced annular noise attenuating baffles, each baffle having an outer circumference and an inner circumference defining a central aperture, the central apertures of each baffle collectively defining an axial outlet flow passage to said outlet aperture;
the baffles defining a series of axially spaced generally annular partial flow passages such that air flowing through said silencer is initially split between said flow passages and then merges into the axial outlet passage;
wherein the dimensions of the annular partial flow passages varies so that the velocity of air flow through the passages is greater for passages closer to the axial outlet aperture.
Advantages resulting from both the first and second aspects of the invention are described further below. Preferred embodiments of the invention combine both the first and second aspects of the invention.
Preferably each of the annular flow passages curves radially inwards from its outer to its inner circumference with a curvature which is initially tangential to a radial plane of said axis and which curves away from said plain towards its inner circumference in a direction towards the axial outlet.
Each of the annular flow passages may have a radius of curvature which increases towards the inner circumference of each respective flow passage.
In preferred embodiments of the invention the axial width of successive annular partial flow passages increases progressively along the axis of the housing towards the axial outlet aperture to provide said increasing velocity of air flow through the passages.
It may be possible to group the passages in size so that two or more adjacent passages have the same axial width which differs from that of a neighbouring passage or group of passages. However, the axial width of each annular partial flow passage preferably differs from that of its immediate neighbour or neighbours.
Silencers according to the present invention may be part of a combined air silence/filter.